The present invention relates to illumination sources for an electrophotographic reproduction device and, more particularly, to a vacuum fluorescent lamp utilized as a document exposure lamp.
Conventional illumination sources used in electrophotographic reproduction devices such as copiers and printers are typically cylindrical low pressure mercury vapor fluorescent lamps. These lamps generally have an excellent spectral match to the photoreceptor, are electrically efficient and moderate in cost. The lamps also have attendant disadvantages, however; the primary ones being temporal and spatial non-uniformity in illumination along the lamp axis, and slow turn-on time.
It is known in the art to utilize a fluorescent-type lamp in which the mercury is removed and a wire anode is disposed axially within the evacuated tube envelope to provide a source of electron emission. As disclosed in a Matsushita Patent Publication No. 56-73970, laid open on June 19, 1981, a glass tube, retained under high vacuum, has a phosphor coated anode extending axially along the tube interior, which when excited, and in conjunction with a control grid and tungsten filament wire, provides an efficient uniform light emission directed towards a surface to be illuminated. The particular configuration used permits a shorter lamp length to be used since there is absent the normal illumination end drop-off associated with conventional lamps. The power requirements are also reduced for this configuration.
U.S. Pat. No. 3,149,262 also discloses a vacuum fluorescent lamp which has a coating on its inner wall, a transparent conductive material upon which is formed a layer of cathode luminescent phosphor. Axially disposed within the lamp is a cathode metallic sleeve. The advantages of this type of lamp are described as faster starting and greater brillance.
These prior art vacuum fluorescent lamps have retained the cylindrical configuration characterizing the conventional fluorescent lamps. Because of this construction, adequate illumination of a document at an image plane is practically realized only by using two lamps, one on each side of an incremental scanning strip area. Alternatively, a single lamp on one side of the scan strip and a reflector on the opposite side provide the required illumination level.
The present invention is directed to a vacuum fluorescent lamp which has a flat geometry, e.g. the lamp envelope has a planar, rather than a cylindrical, surface. This permits the lamp to be placed in a closer location to an object to be illuminated then is possible with a tubular lamp, thereby increasing illumination efficiency. The lamp is also adapted to permit the document image to be viewed through the lamp itself.
More particularly, the invention is directed towards a document illumination device for illuminating a document at an object plane, the device comprising:
a vacuum fluorescent lamp having a planar envelope positioned in close proximity to said object plane, said lamp adapted to illuminate said object plane through an interfacing surface and to transmit a reflected object image through the lamp surface opposite said interfacing surface.
In a preferred embodiment, the lamp envelope has a rectangular configuration.